... ... I'm not sure how useful this is. Give me an example where you want to use this in place of, say, a vampire.
Well, a vampire only heals your current HP, this would actually give you extra max HP if used well.
I.e. its almost strictly better than a vampire. This would be quite useful for farming since the amount of
you win is directly proportional to your max HP.
Its also much gentler on your creatures as opposed to liquid shadow... I.e you could get back 27 max HP and healing out of a Black dragon without causing it to eventually die of poison.
Lastly, since it is also draining max HP from your opponent, it is a very good way to deal with decks that use healing, and makes a wonderful supplement to shards of void.
It also provides an amusing metagame counter for lobotomizers.